An example of an engine in which a turbomachine is used is the SABRE engine developed by Reaction Engines Limited, of Oxfordshire, United Kingdom.
Such an engine is for powering applications such as a single-stage-to-orbit spaceplane such as the SKYLON aircraft developed by Reaction Engines Limited. The SABRE engine has two modes of operation. At lower altitudes, the engine operates by expanding an on-board store of helium in a gaseous helium circuit past a turbine of a turbo-compressor to drive a compressor of the turbo-compressor to compress intake atmospheric air. The compressed air is mixed with hydrogen from an on-board store of liquid hydrogen and the resulting mixture is combusted and then exhausted through a nozzle to provide thrust. At higher altitudes and outside the atmosphere for transport to orbital areas, the engine operates in a full rocket mode. In this mode, instead of taking in atmospheric air, the engine mixes oxygen from an on-board store of liquid oxygen with the hydrogen, and combusts the mixture which is then expanded and exhausted through a rocket nozzle to provide thrust. The turbo-compressor is not used in rocket mode.
One problem to address in an arrangement such as this is the leakage of helium from the helium circuit. Such leakage is undesirable as it requires a larger store of on-board helium than would be the case if there were no leakage or less leakage. Storing less rather than more helium is desirable in minimising size and weight of an aircraft to which an engine such as this is fitted. It is extremely difficult to prevent leakage of fluid such as helium gas past a shaft bearing. In practice there will always be some leakage.
The present arrangement aims to, in effect, minimise leakage by recapturing and re-using leaked working fluid. Alternatively, the invention aims to provide a useful turbomachine.